Currently, an optical fiber used in a popularized optical fiber communication system has a configuration in which the outer circumference of a core is covered with a clad, and an optical signal is propagated in the core to cause information to be transmitted. In recent years, along with the popularization of the optical fiber communication system, the amount of information to be transmitted by the optical fiber drastically increases. In response to such increase in amount of information to be transmitted, as many as tens to hundreds of optical fibers are used in the optical fiber communication system to perform a large-capacity optical communication.
In order to reduce the number of optical fibers in the optical fiber communication system, it is known that a multicore fiber, in which the outer circumferences of a plurality of cores are covered with a clad, is used to transmit a plurality of signals by light propagated in the respective cores.
Non-Patent Document 1 shown below describes such a multicore fiber. However, according to Non-Patent Document 1 shown below, in an optical fiber in which a plurality of cores are formed in a clad, there is a case in which the cores are optically connected to one another, which results in mutual interference among optical signals propagated in the cores. It is said that effective ways to restrict such interference of the optical signals are enlarging the distances among the cores to prevent optical connection among the cores, heightening the refractive indexes of the cores against the clad to concentrate light on the cores, and making the refractive indexes of the adjacent cores different to prevent optical connection among the cores. Further, according to Non-Patent Document 1 shown below, in a case of a multicore fiber using a plurality of cores whose refractive index difference against the clad is 0.35%, optical interference can be restricted to be small when the center-to-center distance of the cores is 70 μm or larger, and in a case where the adjacent cores have different refractive indexes from one another, optical interference can be restricted to be small when the center-to-center distance of the cores is 35 μm or larger (Non-Patent Document 1).